We conducted a 3-yr study of the population dynamics of the invasive cladoceran Bythotrephes longimanus in Island Lake Reservoir (Minnesota) and compared our results to other human-constructed reservoirs and natural lakes to examine ecological differences of B. longimanus between these ecosystem types and to investigate how reservoirs facilitate invasion of this zooplankter. The maximum and mean densities (n 5 number of individuals) of B. longimanus were larger in Island Lake Reservoir (> 100 n m-3 and 20-57 n m-3, respectively) and in most other reservoirs than in natural lakes. Island Lake Reservoir and most other reservoir populations also exhibited multiple (up to six) cycles of growth and collapse per season, and performed extensive diel vertical migrations (DVM) to near the lake bottom during the day. Multiple lines of evidence indicate that the population dynamics of B. longimanus in Island Lake Reservoir were governed by relaxed top-down control by fish predation and strong bottom-up control by food availability. Developmental stage-specific death rates predicted from a matrix model were density dependent, aligned with predictions of bottom-up control (e.g., largest for the smallest instars, and correlated with food resources), and were inconsistent with anticipated prey demands by fishes. The reproductive and DVM trends of B. longimanus also indicated food stress and abated top-down control, respectively. We propose a mechanistic basis for invasion success in reservoirs by linking several limnological attributes, including chlorophyll a, water level fluctuation, light penetration, and thermal conditions to increased growth potential of B. longimanus.